Wideband fading, also known as multipath fading, adversely affects the throughput and spectral efficiency of a wireless communication system. When data is transmitted from a transmitter to a receiver via a radio propagation channel, the fading introduced by the radio propagation channel needs to be equalized by the receiver in order for it to recover the transmitted data. Orthogonal Frequency Division Multiplexing (OFDM) is widely used in modern communication due to its inherent robustness to wideband fading. OFDM effectively converts a wide bandwidth channel into a set of sub-channels of narrow bandwidth, which are used to transmit data. These sub-channels, also referred to as subcarriers, experience narrowband fading. Equalization of narrowband fading requires substantially lower computational complexity. With OFDM, in the presence of multipath fading, each subcarrier typically experiences a different level of fading. Data transmitted on different subcarriers are thus received with different reliabilities. For example, data received on a subcarrier with higher fading (i.e., attenuation) is more likely to be in error relative to data received on a subcarrier with lower level of fading (i.e., attenuation).
Channel coding is employed to exploit this disparity in the subcarrier channel quality. The transmitter applies channel coding to the data before mapping it onto OFDM subcarriers. The receiver employs a channel decoder to recover the information after performing OFDM demodulation. A soft decoder is able to process received signal reliability values thus taking into account channel reliabilities corresponding to each subcarrier. Channel coding combined with soft decision decoding plays a critical role in OFDM systems—not only providing coding gain but also exploiting frequency diversity, and thus providing diversity gain.
Diversity gain can be better exploited by applying a diversity technique, in addition to channel coding. A simple yet very effective way of obtaining frequency diversity gain in OFDM systems is through the application of Multi-Carrier Quadrature Amplitude Modulation (MC-QAM). In MC-QAM based OFDM systems, a set of QAM symbols are transformed and mapped onto a set of OFDM subcarriers. Note that this is in contrast to conventional OFDM systems, where individual QAM symbols are mapped to individual OFDM subcarriers. MC-QAM Modulator transforms sets of QAM symbols such that the MC-QAM can recover the transmitted QAM symbols even in cases when one of the set of subcarriers to which the set of QAM symbols are mapped are severely attenuated by the radio propagation channel.
FIG. 1 (Prior Art) is a block diagram of a transmitter 11 and a receiver 12 in a traditional OFDM system 10. The main functional blocks of a typical OFDM transmitter 11 includes a de-multiplexer, a plurality of channel encoders, a plurality of QAM mappers, an interleaver, and an Inverse Fast Fourier Transform (IFFT) & Cyclic Prefix (CP) insertion module. At the transmitter side, input data 14 (e.g., a sequence of bits) is split into one or more parallel bit-streams by the de-multiplexer. Each bit-stream is encoded into different coding blocks by each of the plurality of channel encoders. Each encoded bit-stream is then mapped onto QAM symbol constellations points by each of the plurality of QAM mappers, resulting in a stream of QAM symbols. The QAM symbols belonging to the different coding blocks are then interleaved by the interleaver. Finally, IFFT is applied and a CP is inserted such that data signal 15 is transmitted via radio propagation channel 13.
The main functional blocks of a corresponding OFDM receiver 12 includes a CP removal & Fast Fourier Transform (FFT) module, a de-interleaver, a plurality of QAM de-mappers, a plurality of channel decoders, and a multiplexer. At the receiver side, the CP is removed from the received data signal 16 and FFT is applied. The signal is then de-interleaved into one or more streams corresponding to one or more coding blocks by the de-interleaver. Each stream is de-mapped by each of the plurality of QAM de-mappers, and then decoded by each of the plurality of channel decoder. Finally, the one or more streams are multiplexed into a sequence of output bits as output data 17 by the multiplexer.
In traditional OFDM system 10, radio propagation channel 13 contains a set of subcarriers, and each QAM symbol is mapped to a corresponding subcarrier based on some fixed or predetermined rules. Adaptive or dynamic mapping rule, however, would be desirable because it improves the overall performance of the channel link at a marginal additional cost in computational complexity and signaling overhead.